Negative active material for rechargeable lithium battery and rechargeable lithium battery

ABSTRACT

Disclosed is a negative active material for a rechargeable lithium battery including ultra-fine particles comprising an element which is capable of alloying with lithium. The particles have a diameter of 1 nm to 200 nm, a Raman shift of 480 cm −1  to 520 cm −1  measured by Raman Spectroscopy, and a full width at half-maximum of 10 cm −1  to 30 cm −1 .

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority of Japanese application No.2003-447 filed in the Japan Patent Office on Jan. 6, 2003 and Koreanapplication No. 2004-263 filed in the Korean Intellectual PropertyOffice on Jan. 5, 2004, the entire disclosures of which are incorporatedhereinto by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to a negative active material for arechargeable lithium battery and a rechargeable lithium batterycomprising the same, and more particularly, to a negative activematerial for a rechargeable lithium battery exhibiting good cycle lifecharacteristics.

BACKGROUND OF THE INVENTION

[0003] Although research to develop a negative active material having ahigh capacity based on metallic materials such as Si, Sn, and Al hasactively been undertaken, such research has not yet succeeded inapplying metals to a negative active material. This is mainly due toproblems with the deterioration of the cycle life characteristics as aresult of alloying of the metallic materials such as Si, Sn, and Al withlithium during charge and discharge, because the alloy expands andcontracts in volume, which produces excessively small micro-particles.

[0004] In order to attempt to solve these problems, an amorphous ormicro-crystalline Si foil obtained from CVD and a sputtering procedureas a negative active material metal has been suggested in JapanesePatent Laid-Open Publication No. 2002-83594. The amorphous Si does notconvert into micro-particles and gives improved cycle lifecharacteristics during repeated charge and discharge, because the volumeexpansion of an alloy of the amorphous Si and lithium is smaller thanthat of a crystalline Si.

[0005] However, a larger capacity than the conventional graphitenegative electrode requires a thick Si foil which requires a longformation time and high cost, and decreases conductivity, deterioratingbattery performance.

[0006] Thus, the bulk crystalline Si is pulverized through mechanicalpulverization under a high shear force, so that the crystalline latticeof Si is distorted to convert it into an amorphous state and to minimizean average diameter thereof, thereby obtaining amorphous Si powder.

[0007] The amorphous Si powder, however, has a wide diameterdistribution between about several hundred nm to 1 μm, and the macroparticles at about 1 μm deteriorate the cycle life characteristicsbecause they are severely expanded and contracted which minimizesparticle size during the charge and discharge.

[0008] It has also been attempted to use a mixture of mechanicallypulverized Si powder and graphite. However, the mechanically pulverizedSi powder has a wide diameter distribution, and it contains macroparticles with a diameter of 1 μm in which volume expansion andshrinkage largely occurs, resulting in the deterioration of the negativeelectrode.

SUMMARY OF THE INVENTION

[0009] It is an aspect of the present invention to provide a negativeactive material for a rechargeable lithium battery in which volumeexpansion and contraction does not occur.

[0010] It is another aspect of the present invention to provide anegative active material exhibiting good cycle life characteristics.

[0011] It is still another aspect of the present invention to provide arechargeable lithium battery including the novel negative activematerial.

[0012] These and other objects may be achieved by a negative activematerial for a rechargeable lithium battery having a diameter of 1 nm to200 nm, a Raman shift of 480 cm⁻¹ to 520 cm⁻¹ measured by RamanSpectroscopy, and a full width at half-maximum of 10 cm⁻¹ to 30 cm⁻¹,and including micro-particles including an element which is capable ofalloying with lithium. Preferably, the element is Si.

[0013] The negative active material includes micro-particles obtainedfrom an evaporation procedure under a gas atmosphere, and themicro-particles have a narrow diameter distribution of between 1 and 200nm and a maximum diameter of 200 nm. The narrow diameter distributionfor the micro-particles causes them to have a different crystallinestructure from Si. An alloy formed from the micro-particles and lithiumdoes not expand during charge and discharge, thereby exhibiting goodcycle life characteristics.

[0014] The micro-particles include at least one of isolatedmicro-particles, linear-linked micro-particles, and agglomeratedmicro-particles. The isolated micro-particles, linear-linkedmicro-particles, and agglomerated micro-particles preferably have adiameter of 1 to 200 nm.

[0015] The negative active material has a Raman Shift of 480 cm⁻¹ to 520cm⁻¹ measured by Raman Spectroscopy, and a full width at half-maximum of10 cm⁻¹ to 30 cm⁻¹. The negative active material has an amorphous form.These physical properties help prevent volume expansion during chargeand discharge, and improvement of the cycle life characteristics.

[0016] The present invention also provides a rechargeable lithiumbattery including the negative active material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] A more complete appreciation of the invention, and many of theattendant advantages thereof, will be readily apparent as the samebecomes better understood by reference to the following detaileddescription when considered in conjunction with the accompanyingdrawings, wherein:

[0018]FIG. 1 is a schematic diagram showing micro-particles included ina negative active material according to one embodiment of the presentinvention;

[0019]FIG. 2 is a schematic diagram showing micro-particles included ina negative active material according to another embodiment of thepresent invention, as a side (perspective) view;

[0020] FIGS. 3(a) and 3(b) are schematic diagrams showingmicro-particles included in a negative active material according toother embodiments of the present invention; and

[0021]FIG. 4 illustrates a battery made according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0022] A negative active material of the present invention is obtainedfrom an evaporation process under a gas atmosphere, and includesmicro-particles of an element which is capable of alloying with lithium.The negative active material micro-particles have a diameter of 1 nm to200 nm. Preferred elements include Si, Pb, Al, and Sn, and mostpreferably Si.

[0023] When a rechargeable lithium battery including the negative activematerial is charged, lithium transfers from a positive electrode to thenegative electrode to form an alloy of lithium and the micro-particlesin the negative electrode. The alloyed micro-particles do not causeexpansion of the volume thereof, thereby improving the cycle lifecharacteristics.

[0024] The reason the volume expansion does not occur is the very smalldiameter and the narrow diameter distribution for the micro-particles.

[0025] The micro-particles have several forms, such as isolatedmicro-particles as shown in FIG. 1, linear-linked micro-particles asshown in FIG. 2, or agglomerated micro-particles as shown in FIGS. 3(a)and 3(b). The isolated micro-particles consist of single particles, butthe linear-linked and agglomerated micro-particles consist of aplurality of nano-particles. As shown in FIGS. 2, 3(a), and 3(b), it isunnecessary for the nano-particles to have the same diameter. Thenegative active material of the present invention includes at least oneof isolated micro-particles, linear-linked micro-particles, andagglomerated micro-particles.

[0026] The micro-particles preferably have a diameter of 1 to 200 nm,regardless of their form. The diameter is the length in the direction oflinear-linking in the linear-linked micro-particles, and in theagglomerated micro-particle the diameter is defined as the largestdiameter thereof.

[0027] The isolated micro-particles, the linear-linked micro-particles,and the agglomerated micro-particles do not cause volume expansionbecause they have a narrow particle distribution and a very smallparticle size.

[0028] The negative active material preferably has a Raman Shift of 480cm⁻¹ to 520 cm−1 measured by Raman Spectroscopy, and a full width athalf-maximum of 10 cm⁻¹ to 30 cm⁻¹.

[0029] Generally, a Raman Shift is lower for amorphous material than forcrystalline material. For example, crystalline Si has a Raman Shift ofover 520 cm⁻¹, and amorphous Si has a lower shift. In addition, the peakis broader for amorphous Si compared to crystalline Si. The physicalproperties help prevent volume expansion and improve the cycle lifecharacteristics.

[0030] Alternatively, as the negative active material, a material isobtained by attachment of the micro-particles to a graphite surface.

[0031] According to one embodiment of the present invention, arechargeable lithium battery includes a negative electrode with thenegative active material described, a positive electrode, and anelectrolyte.

[0032] The negative electrode may be produced, for example, bysolidifying the negative active material of the aggregate into a sheetshape by adding a binder. The binder binds the aggregate of ultra-fineparticles. The aggregate may be solidified into a pellet having acolumnar, discoid, lamellar, or cylindrical shape.

[0033] While the binder may be composed of either an organic or aninorganic material, it should be distributed and dissolved in a solventtogether with the micro-particles, and bind each of the micro-particlesafter removing the solvent. Alternatively, it may be one capable ofbeing solidified by, for example, press solidification, together withthe ultra-fine particles, and binding each into the aggregate. Suchbinders may include a vinyl-based resin, a cellulose-based resin, aphenyl resin, a thermoplastic resin, a thermosetting resin, or similarbinders. Specific examples include polyvinylidene fluoride,polyvinylalcohol, carboxymethyl cellulose, or butylbutadiene rubber.

[0034] The negative electrode of the present invention may furtherinclude a conductive agent such as carbon black, in addition to thenegative active material and the binder.

[0035] The positive electrode may include a positive active materialcapable of intercalating and deintercalating lithium ions. Positiveactive materials may be exemplified as organic disulfide compounds andorganic polysulfide compounds such as LiMn₂O₄, LiCoO₂, LiNiO₂, LiFeO₂,V₂O₅, TiS, and MoS. The positive electrode may further include a bindersuch as polyvinylidene fluoride, and a conductive agent such as carbonblack.

[0036] The positive electrode and the negative electrode may befabricated by coating the positive electrode or the negative electrodeon a current collector of a metal foil to form a sheet.

[0037] The electrolyte may include an organic electrolyte capable ofdissolving the lithium salt in a non-protonic solvent. The non-protonicsolvent may include, but is not limited to, propylene carbonate,ethylene carbonate, butylene carbonate, benzonitrile, acetonitrile,tetrahydrofuran, 2-methyl tetrahydrofuran, γ-butyrolactone, dioxolan,4-methyl dioxolan, N,N-dimethyl formamide, dimethyl acetamide, dimethylsulfoxide, dioxane, 1,2-dimethoxyethane, sulfolane, dichloroethane,chlorobenzene, nitroheptane, dimethylcarbonate, methyl ethyl carbonate,diethylcarbonate, methylpropyl carbonate, methyl isopropyl carbonate,ethyl butyl carbonate, dipropyl carbonate, diisopropyl, carbonate,dibutyl carbonate, diethylene glycol, dimethyl ether, or mixturesthereof. Preferably, it includes any one of propylene carbonate,ethylene carbonate (EC), butylene carbonate, dimethyl carbonate (DMC),methylethyl carbonate (MEC), or diethyl carbonate (DEC).

[0038] Useful lithium salts include LiPF₆, LiBF₄, LiSbF₆, LiAsF₆,LiClO₄, LiCF₃SO₃, Li(CF₃SO₂)₂N, LiC₄F₉SO₃, LiSbF₆, LiAlO₄, LiAlCl₄,LiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂) (wherein x and y are naturalnumbers), LiCl, or Lil, or a mixture thereof, and preferably LiPF₆ orLiBF₄.

[0039] In addition, the electrolyte may include any conventional organicelectrolyte known for fabricating a lithium battery.

[0040] The electrolyte may also include a polymer electrolyte in whichthe lithium salt is mixed with a polymer such as PEO or PVA, or one inwhich an organic electrolyte is impregnated in a high-swelling polymer.

[0041] According to the present invention, the lithium rechargeablebattery may further include any other material, as required, in additionto the positive electrode, the negative electrode, and the electrolyte.For example, a separator separating the positive electrode from thenegative electrode may be included.

[0042] The negative active material according to one embodiment of thepresent invention may be obtained from an evaporation procedure under agas atmosphere. The evaporation procedure includes injecting an inertgas into a vacuum bath and colliding an evaporated molecule against theinert gas molecule. The evaporated molecule is obtained from evaporationor sublimation by heating various materials. The resulting material isslowly cooled, thereby aggregating the molecules to obtainmicro-particles powder and to recover them.

[0043] According to one embodiment of present invention, the inert gasis injected into a vacuum bath under a pressure of 1×10⁻³ Pa to 1×10⁻⁴Pa. Thereafter, Si is evaporated by heating a silicon ingot or siliconpowder through arc discharge under the inert gas atmosphere, which iscontrolled to a back pressure of 1×10⁻⁴ Pa to 5×10⁶ Pa. The evaporatedsilicon molecules collide with the inert gas molecules and are slowlycooled. At this time, the molecules are aggregated to form ultra fineparticles and to recover them.

[0044] The inert gas may be argon, helium, or other gases such asnitrogen which do not react with silicon.

[0045] The heating procedure may be performed by arc discharge,inductive heating, laser heating, resistance heating or electron gunheating. Generally, the heating temperature is set to 100 to 200° C.higher than the melting point of the material to be heated. If thetemperature is lower, it is difficult to evaporate, and if thetemperature is higher, amorphous material cannot be appropriately formedbecause cooling is difficult. For Si, the heating temperature ispreferably 1550 to 1700° C.

[0046] Such a slow cooling process allows formation of amorphous siliconin which silicon molecules are disorderly aggregated. The amorphoussilicon has a diameter of 1 nm to 200 nm, a Raman Shift of 480 cm⁻¹ to520 cm⁻¹, and a full width at half-maximum of 10 cm⁻¹ to 30 cm⁻¹.

[0047] An example of a lithium-sulfur battery according to the inventionis shown in FIG. 4. The lithium-sulfur battery 1 includes a positiveelectrode 3, a negative electrode 4, and a separator 2 interposedbetween the positive electrode 3 and the negative electrode 4. Thepositive electrode 3, the negative electrode 4, and the separator 2 arecontained in a battery case 5. The electrolyte is present between thepositive electrode 3 and the negative electrode 4.

[0048] The following examples further illustrate the present inventionin detail, but are not to be construed to limit the scope thereof.

[0049] Preparation of Negative Active Material

EXAMPLE 1

[0050] Silicon which was previously presented in a vacuum bath washeated to 1700° C. through arc heating under a helium atmosphere of5×10⁴ Pa to generate silicon atmosphere. The generated siliconatmosphere was cooled under the helium atmosphere to aggregate and toform ultra-fine particles. The ultra-fine particles were adhered on aninner surface of the vacuum bath. The procedure was continuouslyperformed for 4 hours to prepare Si ultra-fine particles as a negativeactive material.

[0051] Diameters of the obtained particles were measured using anelectronic microscope, and found to be 10 nm to 200 nm, and they wereseen to be configured as the isolated ultra-fine particles as shown inFIG. 1, the linear-linked ultra-fine particles as shown in FIG. 2, andthe agglomerated ultra-fine particles as shown in FIG. 3. A Raman shiftby Raman Spectroscopy showed a peak at about 500 cm⁻¹ and a full widthat half-maximum of 15 cm⁻¹.

COMPARATIVE EXAMPLE 1

[0052] Silicon macro powder with an average diameter of 1 μm waspulverized with a bead mill with zirconia beads for 24 hours to preparea silicon powder as a negative active material. The diameter of thesilicon powder was checked with an electronic microscope, and the resultwas an average diameter of about 250 nm. However, the silicon powder hadparticles of about 0.9 μm. A Raman shift by Raman Spectroscopy showed apeak at about 490 cm⁻¹, and a full width at half-maximum of 40 cm⁻¹.

COMPARATIVE EXAMPLE 2

[0053] Silicon powder with an average diameter of 1 μm was used as anegative active material. A Raman shift by Raman Spectroscopy showed apeak at about 520 cm⁻¹, and a full width at half-maximum of 9 cm⁻¹.

[0054] Fabrication of Lithium Cell

[0055] 70 parts by weight of each of the negative active materialsaccording to Example 1 and Comparative Examples 1 and 2, 20 parts byweight of a graphite powder with an average diameter of 2 μm as aconductive agent, and 10 parts by weight of a polyvinylidene fluoridebinder were mixed in N-methyl pyrrolidone to prepare a slurry. Theslurry was coated on a copper foil with a thickness of 14 μm and driedfollowed by pressing, thereby producing a negative electrode with athickness of 80 μm. The negative electrode was cut in circles having adiameter of 13 mm. Each negative electrode was placed in a can with apolypropylene separator, the lithium metal counter electrode, and anelectrolyte of 1 mole/L of LiPF₆ in a mixed solution of ethylenecarbonate: dimethyl carbonate, and diethyl carbonate in a 3:3:1 volumeratio to fabricate coin-type lithium half cells.

[0056] Properties of Negative Active Material

[0057] The negative active material according to Example 1 had aparticle diameter of 10 nm to 200 nm, whereas that according toComparative Example 1 had an average particle diameter of about 250 nmwhich is larger than the diameter of Example 1, and had macro particleswith a diameter of 0.9 μm which were not included in the negative activematerial according to Example 1.

[0058] Such differences in Example 1 and Comparative Example 1 arecaused by the differences in the preparation thereof. According to theinvention, negative active material particles with a smaller diameterand a narrower diameter distribution can be achieved than when siliconpowder is obtained from mechanical pulverization of silicon macropowder. Mechanical pulverization tends to result in diameters of about 1μm which causes the negative active material particles to form withlarge diameters and a wide diameter distribution.

[0059] The negative active material according to Example 1 exhibited aRaman Shift of 500 cm⁻¹, and a full width at halt-maximum of 15 cm⁻¹full which indicated that it is amorphous. That of Comparative Example 2exhibited a Raman Shift of 520 cm⁻¹, and a full width at half-maximum of9 cm⁻¹ full which indicated a relatively higher crystallinity than thataccording to Example 1. The negative active material according toComparative Example 1 exhibited a Raman Shift of 490 cm⁻¹, and a fullwidth at half-maximum of 40 cm⁻¹ full which indicated relatively lowercrystallinity than that according to Example 1, and it was amorphous.The result according to Comparative Example 1 is considered to occurbecause mechanical pulverization causes distortion of the crystallinityof silicon.

[0060] Properties of Lithium Cell

[0061] The discharge capacity for 1 cycle (initial discharge capacity)and the discharge capacity retention at the 10^(th) cycle compared tothe discharge capacity at the 1st cycle are shown in Table 1. TABLE 1Discharge capacity (initial discharge capacity) Discharge capacity(mAh/g) retention (%) Example 1 1750 90 Comparative Example 1 1870 68Comparative Example 2 2350 20

[0062] It is evident from Table 1 that the initial discharge capacity inExample 1 is lower than those of Comparative Examples 1 and 2, but thedischarge capacity retention is higher than those in ComparativeExamples 1 and 2. These results are believed to occur for a number ofreasons.

[0063] The relatively low crystallinity of the negative active materialaccording to Example 1 allows a reduced volume expansion of particlesduring charging, thereby preventing the deterioration of the battery,and the ultra-fine particles with a diameter of 200 nm or less haveunique properties, that is, the size effect, compared tohigh-crystalline Si. Furthermore, the negative active material accordingto Example 1 has a different atomic coordination than thehigh-crystallinity Si because it is obtained from the coagulation ofsilicon vapor.

[0064] The negative active material for a rechargeable lithium batteryaccording to the present invention includes ultra-fine particlesobtained from evaporation under a gas atmosphere, and the ultra-fineparticles have a narrow diameter distribution of 1 to 200 nm and amaximum diameter of 200 nm. Even though the ultra-fine particles alloywith lithium during charge and discharge, volume expansion does notoccur. Thus, the negative active material exhibits good cycle lifecharacteristics.

[0065] While the present invention has been described in detail withreference to the preferred embodiments, those skilled in the art willappreciate that various modifications and substitutions can be madethereto without departing from the spirit and scope of the presentinvention as set forth in the appended claims.

What is claimed is:
 1. A negative active material for a rechargeablelithium battery comprising: ultra-fine particles comprising an elementwhich is capable of alloying with lithium, wherein each particle has adiameter between 1 nm and 200 nm, a Raman shift of 480 cm⁻¹ to 520 cm⁻¹measured by Raman Spectroscopy, and a full width at half-maximum of 10cm⁻¹ to 30 cm⁻¹.
 2. The negative active material of claim 1, wherein theultra-fine particles are prepared by evaporation under a gas atmosphere.3. The negative active material of claim 1, wherein the ultra-fineparticles include Si.
 4. The negative active material of claim 1,wherein the ultra-fine particles are selected from the group consistingof isolated ultra-fine particles, linear-linked ultra-fine particles,agglomerated ultra-fine particles, and combinations thereof.
 5. Arechargeable lithium battery comprising: a negative electrode comprisinga negative active material comprising ultra-fine particles comprising anelement which is capable of alloying with lithium, wherein each has adiameter between 1 nm and 200 nm, a Raman shift of 480 cm⁻¹ to 520 cm⁻¹measured by Raman Spectroscopy, and a full width at half-maximum of 10cm⁻¹ to 30 cm⁻¹; a positive electrode comprising a positive activematerial; and an electrolyte.
 6. The rechargeable lithium battery ofclaim 5, wherein the ultra-fine particles are prepared by evaporationunder a gas atmosphere.
 7. The rechargeable lithium battery of claim 5,wherein the ultra-fine particles include Si.
 8. The rechargeable lithiumbattery of claim 5, wherein the ultra-fine particles are selected fromthe group consisting of isolated ultra-fine particles, linear-linkedultra-fine particles, agglomerated ultra-fine particles, andcombinations thereof.